After drilling, the panel is passed through a process which deposits a fine layer of highly conductive carbon into the drilled holes. This carbon layer acts as a conductor and allows copper to plate the hole wall during the later copper plating stage, providing conductivity in a Plated Through Hole (PTH) or multi-layer board from one side to the other and to any internal layers where electrical connectivity is required. The copper surfaces of the drilled panel are first brushed to key them for application of a photo-resist at the lamination stage. The panels are transferred into a fully automatic horizontal metallistaion line where they pass through several pre-treatment stages including cleaners and an ultrasonic bath. The ultrasonic bath removes any fine particles which may reside in the holes and ensures good preparation for carbon deposition. The carbon process floods the surface and holes of the panel with fine particles of carbon. The panels are then dried to complete the adhesion process and passed through a micro-etch bath which removes any excess carbon deposits and further keys the copper surface for the lamination process.
Lamination is the process of applying a UV sensitive photo-resist to the exposed copper faces of a drilled and metallised panel in readiness for the primary imaging stage. This is carried out on a hot roll laminator which, through the process of heat and pressure, ensures that the photo-resist adheres to the contours of the brushed copper panel.
Primary Imaging and Developing:
Primary imaging is the exposure of the previously prepared photo-tool image of the circuit layout onto the UV sensitive photo-resist. The photo-tool is a high contrast, high resolution lithographic film image of the PCB design, any features of the design which should appear as copper on the final Printed Circuit Board are black and all other areas are clear to allow UV to pass through and onto the UV resist. High capacity UV exposure units are used to provide the specific wavelength of UV light necessary for the imaging process. The photo-tool is alligned to the drilled panel through fixed tooling positions to ensure that the registration between drilled holes and corresponding component and via pads is accurate. The panel is then passed into the exposure unit for processing. Clear areas of the film allow the UV light through and expose the resist, circuit features which are black on the fil prevent the UV light from reaching the resist and these ares remain unexposed. When the panel has been exposed it can be passd on to be developed.
This process removes all unexposed areas of UV photoresist from the panel and leaves all exposed areas intact. Exposed panels are developed in a horizontal process which contains a precise concentration of warm sodium carbonate solution. Development time and temperature are strictly controlled to ensure maximum image resolution and to prevent under or over development of the image. All unexposed areas of photo-resist are removed during the development process which, when completed, leaves a copper image of the circuit features on the panel surface. From this point the panels can be transferred to the electrolytic copper plating stage.